Subject changed from Water splashing to Generate particles when touching a plane

Category set to Enhancement

Given that Heretic/Hexen's mechanism for splashes is game-side and involves mobjs and sprites not present in Doom, this shouldn't be built into libdoom as such. However, particle generators started when touching a plane would be quite useful.

There should be a flag for controlling whether the generator is started only when the contact is begun, or should it be running continuously while the contact is in effect, or when contact ends.